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Communication and Computer Networks The
Lecturer: Ahmed Badri


TCP/IP Model

The first layered protocol model for internetwork communications
was created in the early 1970s and is referred to as the Internet model,
TCP/IP is stand for
Transmission Control Protocol

and Internet Protocol
. It
defines four la
yers that must occur for communications to be successful that
illustrated in figure below :

These protocols, which are implemented on both the sending and receiving
hosts, interact to provide end
end delivery of applications over a network.

A comple
te communication process includes these steps:

1. Creation of data at the application layer of the originating source end


2. Segmentation and encapsulation of data as it passes down the protocol

stack in the source end device

3. Genera
tion of the data onto the media at the network access layer of the


4. Transportation of the data through the internetwork, which consists of

media and any intermediary devices

Communication and Computer Networks The
Lecturer: Ahmed Badri


5. Reception of the data at the network access layer of the

destination end


6. Decapsulation and reassembly of the data as it passes up the stack in the

destination device

7. Passing this data to the destination application at the Application layer of

the destination end device

As applicat
ion data is passed down the protocol stack on its way to be
transmitted across the network media, various protocols add information


to it at each level. This is commonly known as the encapsulation
process. The form that a piece of data takes at an
y layer is called a Protocol
Data Unit (PDU). During encapsulation, each succeeding layer encapsulates
the PDU that it receives from the layer above in accordance with the
protocol being used. At each stage of the process, a PDU has a different
name to ref
lect its new appearance. Although there is no universal naming
convention for PDUs, in this course, the PDUs are named according to the
protocols of the TCP/IP suite.

• Data

The general term for the PDU used at the Application layer

• Segment

rt Layer PDU

• Packet

Internetwork Layer PDU

• Frame

Network Access Layer PDU


A PDU used when physically transmitting data over the medium

Communication and Computer Networks The
Lecturer: Ahmed Badri


As an example of protocol modal are
web server protocol that has four
equivalent to TCP/IP p
rotocols as in this figure :

Communication and Computer Networks The
Lecturer: Ahmed Badri


Application Protocol:

Hypertext Transfer Protocol (HTTP) is a common protocol that governs the
way that a web server and a web client interact. HTTP defines the content
and formatting of the requests and responses exchang
ed between the client
and server.

Transport Protocol:

Transmission Control Protocol (TCP) is the transport protocol that manages
the individual conversations between web servers and web clients. TCP
divides the HTTP messages into smaller pieces, called se
gments, to be sent
to the destination client.

Internetwork Protocol:

The most common internetwork protocol is Internet Protocol (IP). IP is
responsible for taking the formatted segments from TCP, encapsulating them
into packets, assigning the appropriate
addresses, and selecting the best path
to the destination host.

Network Access Protocols

Network access protocols describe two primary functions, data link
management and the physical transmission of data on the media. Data
management prot
ocols take the packets from IP and format them to be
transmitted over the media. The standards and protocols for the physical
media govern how the signals are sent over the media and how they are
interpreted by the receiving clients.

In the web

server example, we can use the TCP/IP model to illustrate
the process of sending an HTML web page to a client.

HTTP, begins the process by delivering the HTML formatted web page
data to the Transport layer. There the application data is broken into TCP

segments. Each TCP segment is given a label, called a header, containing
information about which process running on the destination computer
should receive the message. It also contains the information to enable the
destination process to reassemble the d
ata back to its original format.

Communication and Computer Networks The
Lecturer: Ahmed Badri


The Transport layer encapsulates the web page HTML data within the
segment and sends it to the Internet layer, where the IP protocol is

Here the entire TCP segment is encapsulated within an IP packet, which

adds another label, called the IP header. The IP header contains source and
destination host IP addresses

the IP packet is sent to the Network Access layer Ethernet protocol where
it is encapsulated within a frame header and trailer. Each frame header
contains a source and destination physical address. The physical address
uniquely identifies the devices on the local network. The trailer contains
error checking information.